BANNU, Pakistan (Reuters) - An Islamist militant commander who helped plan an assault on a Pakistani jail on Sunday which freed nearly 400 prisoners said his group had inside information.

Pakistan's Taliban movement, which is close to al Qaeda, said it was behind the brazen assault by militants armed with rocket-propelled grenades and AK-47 assault rifles.

A police official said most of those who escaped from the jail in the northwestern town of Bannu were militants, including

one on death row for trying to assassinate former president Pervez Musharraf.

"We had maps of the area and we had complete maps and plans of the jail as well," the commander, a senior member of the Taliban, told Reuters.

"All I have to say is we have people who support us in Bannu. It was with their support that this operation was successful."

It was not possible to independently verify that account.

But the inspector general for prisons in the northwestern Khyber-Pakhtunkhwa province, Arshad Majeed, did not rule out collusion.

"It is possible that the attackers had help from the inside. We cannot confirm that but we are investigating to see what happened," he said.

The jail break, described as one of the biggest in Pakistan's history, underscored the strategic American ally's struggle to tame militancy over a decade since it joined the U.S.-led "war on terror".

Pakistan's performance against militants came under greater scrutiny after U.S. special forces killed Osama bin Laden in May last year in a Pakistani town where he had apparently been living for years.

Pakistani officials describe bin Laden's long presence in Abbottabad as a security lapse and reject suggestions that members of the military and intelligence service were complicit in hiding him there.

Many Pakistanis believe law enforcement officials are corrupt and inept, and complain the judicial system consistently fails to lock up criminals, including militants who carry out violent attacks.

The prison break could be a psychological blow to security forces following repeated government assertions that security crackdowns have weakened militant groups.

Pakistan is seen as critical to U.S. efforts to stabilise Afghanistan, yet Pakistan faces its own daunting security problems.

The Tehrik-e-Taliban Pakistan (TTP), or Taliban Movement of Pakistan, is seen as the biggest threat, staging suicide bombings and attacks on military compounds.

The Bannu attackers arrived in a convoy of vehicles, blocking off all access points to the jail before firing rocket-propelled grenades at the black, metal gates of the prison and forcing their way in.

They moved through the prison quickly facing little resistance until they found Adnan Rasheed, who took part in one of the attempts to kill Musharraf, and then freed him and 383 others.

"We had set one hour as the target time for our mission. This included entering the jail, breaking out our people, getting back into our vehicles and reaching a safe spot," said the commander, who asked not to be named.

Citing preliminary investigations into the prison break, Majeed said 150 guards were supposed to be on duty when the militants struck, but only 36 were.

Even the ones that showed up to work were poorly equipped to deal with the onslaught by dozens of militants.

Standing outside the prison, where over 900 were held before the escape, constable Firdous Khan showed a Reuters reporter the only AK-47 assault rifle magazine he had.

"This emptied out pretty quickly. Then I was out," he said.

The Taliban commander, based in North Waziristan -- home to some of the world's most dangerous militants -- said 150 fighters, including foreigners, launched the prison assault after months of training.

"We spent 20 million rupees on this mission," he said.

Police officials said 76 escapees were now back in custody.

One of them, Zahir Shah, 29, described the chaos following the attack and why he decided to reject freedom after escaping.

"They took us with them to the main gate. Outside there were many cars. I think 20 cars in all. They shoved me into one of the vehicles and drove off really fast," he said as he trembled outside the prison before turning himself in.

"I could be out in a few years for good behaviour. I don't want to ruin my chances."

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